Ultrasensitive SERS Detection of TNT by Imprinting Molecular Recognition Using a New Type of Stable Substrate

Advancements in nanomaterials for nanosensors: a comprehensive review

Nanomaterials (NMs) exhibit unique properties that render them highly suitable for developing sensitive and selective nanosensors across various domains. This review aims to provide a comprehensive overview of nanomaterial-based nanosensors, highlighting their applications and the classification of frequently employed NMs to enhance sensitivity and selectivity. The review introduces various classifications of NMs commonly used in nanosensors, such as carbon-based NMs, metal-based NMs, and others, elucidating their exceptional properties, including high thermal and electrical conductivity, large surface area-to-volume ratio and good biocompatibility. A thorough examination of literature sources was conducted to gather information on NMs-based nanosensors' characteristics, properties, and fabrication methods and their application in diverse sectors such as healthcare, environmental monitoring, industrial processes, and security. Additionally, advanced applications incorporating machine learning techniques were analyzed to enhance the sensor's performance. This review advances the understanding and development of nanosensor technologies by providing insights into fabrication techniques, characterization methods, applications, and future outlook. Key challenges such as robustness, biocompatibility, and scalable manufacturing are also discussed, offering avenues for future research and development in this field.

Rapid detection of trace nitrobenzene in water via SERS using a portable Raman spectrometer

Nitrobenzene is currently the most widely used explosive substance, and is known for its high toxicity and mutagenicity. It can cause severe environmental and water pollution, posing a risk to public health. Among various explosives analysis methods, surface-enhanced Raman spectroscopy (SERS) has the advantages of fast analysis speed, low detection cost, and easy operation, and has become one of the most promising analytical detection methods. Here, we present a portable and reliable sol-based SERS method for the detection of trace amounts of 2,4,6-trinitrotoluene (TNT) in different water bodies. The Meisenheimer complex formed by nitrobenzene and hydrazine hydrate can assemble on unmodified Au nanoparticles in a sol via Au-N bonds, enabling rapid detection of TNT in seawater, lake water, and tap water using a portable Raman spectrometer. Experimental results show that this SERS method can complete the detection within a few minutes and the detection sensitivity can reach 0.01 mg L-1, which is far lower than China's national standard of no more than 0.5 mg L-1. Furthermore, this method was also successfully applied to detect trace 2,4-dinitrotoluene (2,4-DNT) and picric acid (2,4,6-trinitrophenol) in water, demonstrating its strong applicability for on-site detection of nitrobenzene explosives.

Recent progress in SERS monitoring of photocatalytic reactions

Surface-enhanced Raman spectroscopy (SERS) is a powerful analytical technique renowned for its ultra-high sensitivity. Extensive research in SERS has led to the development of a wide range of SERS substrates, including plasmonic metals, semiconductors, metal organic frameworks, and their assemblies. Some of these materials are also excellent photocatalysts, and by taking advantage of their bifunctional characteristics, the photocatalytic processes that occur on their surface can be monitored in situ via SERS. This provides us with unique opportunities to gain valuable insights into the intricate details of the photocatalytic processes that are challenging to access using other techniques. In this review, we highlight key development in in situ and/or real-time SERS-tracking of photocatalytic reactions. We begin by providing a brief account of recent developments in SERS substrates, followed by discussions on how SERS can be used to elucidate crucial aspects of photocatalytic processes, including: (1) the influence of the surrounding media on charge carrier extraction; (2) the direction of charge carrier transfer; (3) the pathway of photocatalytic activation; and (4) differentiation between the effects of photo-thermal and energetic electrons. Additionally, we discuss the benefits of tip-enhanced Raman spectroscopy (TERS) due to the ability to achieve high-spatial-resolution measurements. Finally, we address major challenges and propose potential directions for the future of SERS monitoring of photocatalytic reactions. By leveraging the capabilities of SERS, we can uncover new insights into photocatalytic processes, paving the way for advancements in sustainable energy and environmental remediation.

A 'sandwich' structure for highly sensitive detection of TNT based on surface-enhanced Raman scattering

Ultra-sensitive detection of 2,4,6-trinitrotoluene (TNT) plays an important role in society security and human health. The Raman probe molecule p-aminothiophenol (PATP) can interact with TNT in three ways to form a TNT-PATP complex. In this paper, a 'sandwich' structure was developed to detect TNT with high sensitivity. Au nano-pillar arrays (AuNPAs) substrates modified by low-concentration PATP through Au-S bonds were acted as capture probe for TNT. Meanwhile, Ag nano-particles (AgNPs) modified by PATP at higher concentration were employed as tags for surface-enhanced Raman scattering (SERS). The formation of the TNT-PATP complex is not only the means by which AuNPAs substrates recognize and capture TNT, but also links the SERS tags to TNT, forming an AuNPAs-TNT-AgNPs 'sandwich' structure. The Raman signal of PATP was greatly enhanced mainly because novel 'hot spots' formed between the AuNPAs and AgNPs of the 'sandwich' structure. The Raman signal of PATP was further amplified by the chemical enhancement effect induced by the TNT-PATP complex formation. Based on this mechanism, the limit of detection (LOD) of TNT was determined from the Raman signal of PATP. The LOD reached 10^-9 mg/mL (4.4 × 10^-12 M), much lower than that suggested by the US Environmental Protection Agency (88 nM). Moreover, TNT was selectively detected over several TNT analogues 2,4-dinitrotoluene (DNT), p-nitrotoluene (NT) and hexogen (RDX). Finally, the 'sandwich' structure was successfully applied to TNT detection in environmental water and sand.

Construction of MOF-shell porous materials and performance studies in the selective adsorption and separation of benzene pollutants

Metastable Cu2O is an attractive material for the architectural design of integrated nanomaterials. In this context, Cu2O was used as the sacrificial agent to form the core-shell structure of Cu2O@HKUST-1 by in situ growth technology. The MOFs with BOPs adsorption property were gathered together by a Cu2O etching method, and the hollow structure of the HKUST-1 shell material with fast BOP adsorption was successfully constructed. The adsorption experiments showed that the HKUST-1 shell has a good adsorption effect on nitrobenzene pollutants in wastewater. The investigation of various factors affecting the adsorption, thermodynamic and kinetic equations was carried out. The adsorption equilibrium was reached within 30 min, and the maximum adsorption capacity was 94.67 mg g-1 at 298 K. The adsorption capacity of nitrobenzene by the HKUST-1 shell is in good agreement with the Freundlich model and the second-order kinetic model. The possible mechanism of adsorption of nitrobenzene by the HKUST-1 shell was discussed. The experimental results suggested that Cu-BTC materials have potential applications for wastewater treatment involving benzene pollutants.

Raman spectroscopy-based sensitive, fast and reversible vapour phase detection of explosives adsorbed on metal–organic frameworks UiO-67

A sensitive, selective, rapid, and reversible detection of explosive molecules in the vapour phase, adsorbed on metal–organic frameworks (MOFs) under ambient laboratory conditions is demonstrated using Raman spectroscopy.

An overview on molecular imprinted polymers combined with surface-enhanced Raman spectroscopy chemical sensors toward analytical applications

Surface-enhanced Raman spectroscopy (SERS) is a powerful and high-speed detection technology. It provides information on molecular fingerprint recognition with ultrahigh sensitive detection. However, it shows poor anti-interference capacity against complex matrices. Molecularly imprinted polymers (MIPs) can achieve specific recognition of targets from complex matrices. Through introducing the MIP separation system, the MIP-SERS chemical sensor can effectively overcome the limitation of complex matrix interference, and further improve the stability of sensors for detection. Herein, the materials and structures of integrated MIP-SERS sensors are systematically reviewed, and its application as a sensor for chemical detection of hazardous substances in environmental and food samples has been addressed as well. To broaden the prospects of application, we have discussed the current challenges and future perspectives that would accelerate the development of versatile MIP-SERS chemical sensors.

Toward the Development of Ultrasensitive Detectors for Environmental Applications: A Kinetic Study of Cr(III) Monitoring in Water Using EDTA and SERS Techniques

We report for the first time kinetic studies on chromium(III) detection in aqueous solution using citrate-capped silver nanoparticles (AgNPs) and the surface-enhanced Raman spectroscopy (SERS) technique. Moreover, we have shown an important effect of adding ethylenediaminetetraacetic acid (EDTA) on the enhancement and the stability of the Raman signal. The origin of the SERS signal was attributed to the coordination of Cr(III) by citrate/EDTA molecules and the formation of hot spots on aggregated AgNPs. Depending on the mixing method of Cr(III) and EDTA with AgNPs, the temporal SERS spectral features reveal a Prout-Tompkins or a Langmuir kinetic detection model. The UV-visible data, the temporal response of the Raman signal, and the scanning electron microscopy analysis have allowed us to elucidate the mechanism of Cr(III) detection. We observed that mixing simultaneously Cr(III), AgNPs, and EDTA leads to the most stable and intense time-dependent SERS signal. The obtained results should open the way to perform kinetic studies on different host-guest interactions in solution using the SERS technique.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

From children's toy to versatile sensor: One-step doping of Play-Doh with primary amino group for explosive detection both on surfaces and in solution

2,4,6-trinitrotoluene (TNT) sensing on surfaces and in solution is an important issue in sensor fabrication for homeland security and environmental protection. Herein, Play-Doh, a modeling material popular for kids, was proposed as a versatile sensor for on-site fluorescent (FL), visual FL (VFL), and colorimetric detection of TNT both on surfaces and in solution after being doped with -NH₂ through a one-step approach. Play-Doh exhibits FL emission due to the main ingredient of flour. After -NH₂ doping, amino-Play-Doh (APD) was utilized to construct a FL sensor based on FL resonance energy transfer and inner filter effect for TNT detection. The advantage of APD was that no additional fluorophore was needed compared with the traditional sensors for FL and VFL analysis. The orange complex visible to the naked eye was also recorded for smartphone-based colorimetric detection of TNT. In both cases, the APD demonstrated good analytical performance for TNT. Finally, APD was successfully utilized for TNT sensing on fingerprints, luggage, and in environmental water samples, respectively. Play-Doh might be a potential sensor for future on-site detection of TNT owing to the merits of being cost-effective and versatile.

A novel SERS selective detection sensor for trace trinitrotoluene based on meisenheimer complex of monoethanolamine molecule

Trinitrotoluene (TNT) is a primary component in chemical explosives, making them a common focus in public safety detection. However, it is very difficult to achieve selective and sensitive detection of the TNT molecule in practical application. In the present study, a simple surface enhanced Raman scattering (SERS) sensing based on monoethanolamine (MEA) - modified gold nanoparticles (Au NPs) was expanded for high selectivity and sensitive detecting of TNT in an envelope, luggage, lake water, and clothing through a quickly sampling and detection process. The monoethanolamine molecule based on Meisenheimer complex lights up ultra-high Raman scattering of a nonresonant molecule on the superficial coat of gold nanoparticles. Using this detection sensor, a molecular bridge can be established to selectively detect trinitrotoluene with a detection limit of 21.47 pM. We were able to rapidly identification trinitrotoluene molecule with a powerful selective over the familiar interfering substances nitrophenol, picric acid, 2,4-dinitrophenol, and 2,4-dinitrotoluene. The outcome in this work supply an efficient solution to the test of trinitrotoluene and to establishing a SERS sensor analytical strategy. The studies have demonstrated that the MEA-Au NPs based SERS sensing can be potentially used in field detection the trace amount of chemical explosives for public security.

Surface enhanced Raman scattering substrate for the detection of explosives: Construction strategy and dimensional effect

Surface-enhanced Raman spectroscopy (SERS) technology has been reported to be able to quickly and non-destructively identify target analytes. SERS substrate with high sensitivity and selectivity gave SERS technology a broad application prospect. This contribution aims to provide a detailed and systematic review of the current state of research on SERS-based explosive sensors, with particular attention to current research advances. This review mainly focuses on the strategies for improving SERS performance and the SERS substrates with different dimensions including zero-dimensional (0D) nanocolloids, one-dimensional (1D) nanowires and nanorods, two-dimensional (2D) arrays, and three-dimensional (3D) networks. The effects of elemental composition, the shape and size of metal nanoparticles, hot-spot structure and surface modification on the performance of explosive detection are also reviewed. In addition, the future development tendency and application of SERS-based explosive sensors are prospected.

Molecular-Imprinting-Based Surface-Enhanced Raman Scattering Sensors

Molecularly imprinted polymers (MIPs) receive extensive interest, owing to their structure predictability, recognition specificity, and application universality as well as robustness, simplicity, and inexpensiveness. Surface-enhanced Raman scattering (SERS) is regarded as an ideal optical detection candidate for its unique features of fingerprint recognition, nondestructive property, high sensitivity, and rapidity. Accordingly, MIP based SERS (MIP-SERS) sensors have attracted significant research interest for versatile applications especially in the field of chemo- and bioanalysis, showing excellent identification and detection performances. Herein, we comprehensively review the recent advances in MIP-SERS sensors construction and applications, including sensing principles and signal enhancement mechanisms, focusing on novel construction strategies and representative applications. First, the basic structure of the MIP-SERS sensors is briefly outlined. Second, novel imprinting strategies are highlighted, mainly including multifunctional monomer imprinting, dummy template imprinting, living/controlled radical polymerization, and stimuli-responsive imprinting. Third, typical application of MIP-SERS sensors in chemo/bioanalysis is summarized from both small and macromolecular aspects. Lastly, the challenges and perspectives of the MIP-SERS sensors are proposed, orienting sensitivity improvement and application expanding.

Qualitative Detection Toward Military and Improvised Explosive Vapors by a Facile TiO2 Nanosheet-Based Chemiresistive Sensor Array

A facile TiO₂ nanosheets-based chemiresistive gas sensor array was prepared to identify 11 kinds of military and improvised explosive vapors at room temperature. The morphology of TiO₂ nanosheets was well-controlled by adjusting the concentration of HF applied during the preparation. Owing to the morphology difference, the TiO₂ nanosheet-based sensors show different response values toward 11 kinds of explosives, which is the basis of the successful discriminative identification. This method owes lots of advantages over other detection techniques, such as the facile preparation procedure, high response value (115.6% for TNT and 830% for PNT) at room temperature, rapid identifying properties (within 30 s for 9 explosives), simple operation, high anti-interference property, and low probability of misinforming, and consequently has a huge potential application in the qualitative detection of explosives.

Controllable fabrication of Ag-NP-decorated porous ZnO nanosheet arrays with superhydrophobic properties for high performance SERS detection of explosives

An efficient hydrophobic condensation surface was developed and used as an ultrasensitive and stable SERS sensor based on ZnO–Ag hybrid mesoporous nanosheet (MNS) arrays for natural explosive sample detection.

Stepwise elucidation of fluorescence based sensing mechanisms considering picric acid as a model analyte

The precise study of fluorescence-based sensing mechanisms and a step-by-step design experiment for the elucidation of the mechanism of sensing for newly designed sensing systems can be ascertained using the presented tutorial review.

Synthesis of corrugated surface AgNWs and their applications in surface enhanced Raman spectroscopy

Among metals, AgNWs are considered to be excellent materials for use in surface enhancement Raman spectroscopic (SERS) sensing due to their superior electrical properties, strong electromagnetic field generation and strong enhancement intensity.

Highly Sensitive and Stable SERS Substrate Fabricated by Co-sputtering and Atomic Layer Deposition

In this study, we develop a facile method to fabricate highly sensitive and stable surface-enhanced Raman scattering (SERS) substrate, which is realized by combining co-sputtering with atomic layer deposition technology. To accomplish the SERS substrate preparation, we firstly utilized co-sputtering silver and aluminum on glass slides to form uniform discontinuous Ag film by removing Al later, which acted as SERS active moiety and presented high sensitivity in glycerin detection. After coating an ultrathin TiO₂ layer via atomic layer deposition (ALD), the samples could further enhance the Raman signal due to the chemical effect as well as the long-range effect of the enhanced electromagnetic field generated by the encapsulated Ag nanoparticles (NPs). Besides, the coated sample could maintain the significant enhancement in air condition for more than 30 days. The high stability is induced by TiO₂ layer, which efficiently prevents Ag NPs from surface oxidation. This highly sensitive and stable SERS substrate might highlight the application of interface state investigation for exploring novel liquid lubricating materials.

Molecularly Imprinted Polymer Materials as Selective Recognition Sorbents for Explosives: A Review

Explosives are of significant interest to homeland security departments and forensic investigations. Fast, sensitive and selective detection of these chemicals is of great concern for security purposes as well as for triage and decontamination in contaminated areas. To this end, selective sorbents with fast binding kinetics and high binding capacity, either in combination with a sensor transducer or a sampling/sample-preparation method, are required. Molecularly imprinted polymers (MIPs) show promise as cost-effective and rugged artificial selective sorbents, which have a wide variety of applications. This manuscript reviews the innovative strategies developed in 57 manuscripts (published from 2006 to 2019) to use MIP materials for explosives. To the best of our knowledge, there are currently no commercially available MIP-modified sensors or sample preparation methods for explosives in the market. We believe that this review provides information to give insight into the future prospects and potential commercialization of such materials. We warn the readers of the hazards of working with explosives.

Manganese-doped carbon quantum dots-based fluorescent probe for selective and sensitive sensing of 2,4,6-trinitrophenol via an inner filtering effect

In the present work, a selective and sensitive method for detecting TNP using manganese doped carbon quantum dots (Mn-CDs) was developed. The Mn-CDs were prepared via a simple hydrothermal method using 1-(2-pyridinylazo)-2-naohthalenol naohthalenol (PAN) and MnCl₂ as precursors. The as-prepared Mn-CDs have UV emission with high quantum yield (83.2%). Because of the strong characteristic absorption of TNP at 356 nm, which has good spectral overlap with the emission peak of Mn-CDs, the fluorescence intensity of Mn-CDs at 360 nm is linearly quenched in the presence of TNP in the concentration range of 0.1-200 μM. The developing assay based on an inner filter effect (IFE) mechanism for detecting TNP is selective, convenient, and shows that the as-prepared Mn-CDs have application prospects for simple and specific analytical chemistry.

Inner Filter Effect and Resonance Energy Transfer Based Attogram Level Detection of Nitroexplosive Picric Acid Using Dual Emitting Cationic Conjugated Polyfluorene

A novel conjugated cationic polyfluorene (polyelectrolyte) derivative, PFBT, was developed by means of simple and cost-effective oxidative coupling polymerization method. PFBT displayed dual state emission in dimethyl sulfoxide (DMSO) as well as in water, a characteristic phenomenon of polyfluorene homopolymers, and tested for nitroexplosive analytes detection to observe a remarkable fluorescence quenching response for picric acid (PA) in the both solvents. The polymer PFBT demonstrated substantial selectivity and ultrasensitivity toward nitroexplosive PA in both the solvents (DMSO and H₂O) with exceptional quenching constant values of 2.69 × 10⁴ and 2.18 × 10⁵ M^-1 and a ultralow limit of detection of 92.7 nM (21.23 ppb) and 0.19 nM (43.53 ppt) in respective solvents. Furthermore, economical portable test strip devices were prepared for easy and fast on-site PA sensing, which can detect up to 0.22 ag level of PA. PA sensing in vapor phase was also established, that could detect up to 42.6 ppb level of PA vapors. Interestingly, the mechanism of sensing in DMSO solvent was attributed to substantial inner filter effect and photoinduced electron transfer, while in H₂O the sensing occurs via possible resonance energy transfer and photoinduced electron transfer, which is exceptional and not reported earlier for a single probe.

Electrospun flexible poly(bisphenol A carbonate) nanofibers decorated with Ag nanoparticles as effective 3D SERS substrates for trace TNT detection

A flexible 3D hybrid PC/Ag surface-enhanced Raman scattering (SERS) substrate was fabricated through the combination of electrospinning and in situ chemical reduction. Due to the rough surface morphology and the intricate 3D structure, a high density of Raman "hotspots" was formed at the junctions of cross-linked nanofibers, resulting in excellent sensitivity to a probe molecule (4-aminothiophenol). The nanofibers were modified with l-cysteine to capture TNT molecules by the formation of a Meisenheimer complex, after which positively charged 4-ATP-labelled AgNPs were introduced to the system, which both generated more hotspots and led to a linear relationship between the TNT concentration and the SERS intensity of the labelled molecules. As a result, a good linear response ranging from 10^-8 to 10^-12 M was achieved, and the detection limit for TNT was as low as 2.05 × 10^-13 M. This strategy demonstrates an ultra-sensitive approach for the detection of trace amounts of TNT, and a promising method for the detection of many other analogous explosives.

In situ preparation of Ag nanoparticles on silicon wafer as highly sensitive SERS substrate

An intensive surface enhanced Raman scattering (SERS) effect is realized by ordered Ag nanoparticles (NPs) in situ grown on silicon wafer directly using (3-aminopropyl) trimethoxysilane (APS) as both the surface modifier and reducing agent. The as-prepared ordered Ag NPs based SERS substrate shows excellent performance in detecting glycerin (an important integration in liquid super lubricating system) as well as conventional Rhodamine 6G (R6G, a kind of dye organic pollutant). The enhancement factor (EF) achieves 4-fold for glycerin and 10-fold for R6G (allowing for detecting as low as 10-11 M aqueous R6G), confirming the high sensitivity. The limited relative standard deviations (RSD) of the enhancement factors are within 15% for both glycerin and R6G, indicating the excellent uniformity. This remarkable progress is ascribed to the advantages of APS in improving adsorption and modulating distribution of Ag NPs on silicon, which results in a large local electric field to enhance the Raman signals. The SEM and UV-visible absorption spectrum characterization verified the contribution of APS in SERS improvement by investigating the influence of APS content and reduction time during the preparation process. All these advances imply that the SERS substrates prepared by Ag NPs in situ grown on silicon wafer have great potential application in real-time interface state tracing and sensitive detection.

Investigation of aromatic hydrocarbon inclusion into cyclodextrins by Raman spectroscopy and thermal analysis

Among various cavitand molecules, cyclodextrins are extensively studied due to their ability to form host-guest complexes with small hydrophobic molecules. Aiming to explore cyclodextrin implementation on the scopes related to the environmental pollution monitoring or remediation, extensive studies for understanding the cyclodextrin-based host-guest complex formation with selected targeted substances are conducted. In this context, two polycyclic aromatic hydrocarbons, naphthalene and fluoranthene as well as toluene as a member of volatile organic compounds, were studied regarding their ability to encapsulate into cyclodextrin cavities. Synthesised complexes were examined by thermogravimetric analysis combined with Raman spectroscopy. The obtained results demonstrated that the size between targeted molecules and the cyclodextrin cavities strongly correlates with its ability to engage in complexation. Thus, this latter parameter plays an important role in the inclusion complex formation as well as in the strength of the interaction between the molecules.

Hierarchically-Designed 3D Flower-Like Composite Nanostructures as an Ultrastable, Reproducible, and Sensitive SERS Substrate

Surface-enhanced Raman spectroscopy (SERS) is an attractive tool in the analytical sciences due to its high specificity and sensitivity. Because SERS-active substrates are only available as two-dimensional arrays, the fabrication of three-dimensional (3D) nanostructures allows for an increased number of hot spots in the focus volume, thus further amplifying the SERS signal. Although a great number of fabrication strategies for powerful SERS substrates exist, the generation of 3D nanostructures with high complexity and periodicity is still challenging. For this purpose, we report an easy fabrication technique for 3D nanostructures following a bottom-up preparation protocol. Enzymatically generated silver nanoparticles (EGNPs) are prepared, and the growth of hierarchically-designed 3D flower-like silica-silver composite nanostructures is induced by applying plasma-enhanced atomic layer deposition (PE-ALD) on the EGNPs. The morphology of these nanocomposites can be varied by changes in the PE-ALD cycle number, and a flower height of up to 10 μm is found. Moreover, the metallized (e.g., silver or gold) 3D nanostructures resulting from 135 PE-ALD cycles of silica creation provide highly reproducible SERS signals across the hydrophobic surface. Within this contribution, the morphological studies, optical properties, as well as the SERS response of these metallized silica-silver composite nanostructures applying vitamin B2 as a model analyte are introduced.

Rational Design of Peptide-Functionalized Surface Plasmon Resonance Sensor for Specific Detection of TNT Explosive

In this study, a rationally-designed 2,4,6-trinitrotoluene (TNT) binding peptide derived from an amino acid sequence of the complementarity-determining region (CDR) of an anti-TNT monoclonal antibody was used for TNT detection based on a maleimide-functionalized surface plasmon resonance (SPR) sensor. By antigen-docking simulation and screening, the TNT binding candidate peptides were obtained as TNTHCDR1 derived from the heavy chain of CDR1, TNTHCDR2 derived from CDR2, and TNTHCDR3 from CDR3 of an anti-TNT antibody. The binding events between candidate peptides and TNT were evaluated using the SPR sensor by direct determination based on the 3-aminopropyltriethoxysilane (APTES) surface. The TNT binding peptide was directly immobilized on the maleimide-functionalized sensor chip surface from N-γ-maleimidobutyryl-oxysuccinimide ester (GMBS). The results demonstrated that peptide TNTHCDR3 was identified and selected as a TNT binding peptide among the other two candidate peptides. Five kinds of TNT analogues were also investigated to testify the selectivity of TNT binding peptide TNTHCDR3. Furthermore, the results indicated that the APTES-GMBS-based SPR sensor chip procedure featured a great potential application for the direct detection of TNT.

Ag Nanorods-Oxide Hybrid Array Substrates: Synthesis, Characterization, and Applications in Surface-Enhanced Raman Scattering

Over the last few decades, benefitting from the sufficient sensitivity, high specificity, nondestructive, and rapid detection capability of the surface-enhanced Raman scattering (SERS) technique, numerous nanostructures have been elaborately designed and successfully synthesized as high-performance SERS substrates, which have been extensively exploited for the identification of chemical and biological analytes. Among these, Ag nanorods coated with thin metal oxide layers (AgNRs-oxide hybrid array substrates) featuring many outstanding advantages have been proposed as fascinating SERS substrates, and are of particular research interest. The present review provides a systematic overview towards the representative achievements of AgNRs-oxide hybrid array substrates for SERS applications from diverse perspectives, so as to promote the realization of real-world SERS sensors. First, various fabrication approaches of AgNRs-oxide nanostructures are introduced, which are followed by a discussion on the novel merits of AgNRs-oxide arrays, such as superior SERS sensitivity and reproducibility, high thermal stability, long-term activity in air, corrosion resistivity, and intense chemisorption of target molecules. Next, we present recent advances of AgNRs-oxide substrates in terms of practical applications. Intriguingly, the recyclability, qualitative and quantitative analyses, as well as vapor-phase molecule sensing have been achieved on these nanocomposites. We further discuss the major challenges and prospects of AgNRs-oxide substrates for future SERS developments, aiming to expand the versatility of SERS technique.

Detecting explosive molecules from nanoliter solution: A new paradigm of SERS sensing on hydrophilic photonic crystal biosilica

We demonstrate a photonic crystal biosilica surface-enhanced Raman scattering (SERS) substrate based on a diatom frustule with in-situ synthesized silver nanoparticles (Ag NPs) to detect explosive molecules from nanoliter (nL) solution. By integrating high density Ag NPs inside the nanopores of diatom biosilica, which is not achievable by traditional self-assembly techniques, we obtained ultra-high SERS sensitivity due to dual enhancement mechanisms. First, the hybrid plasmonic-photonic crystal biosilica with three dimensional morphologies was obtained by electroless-deposited Ag seeds at nanometer sized diatom frustule surface, which provides high density hot spots as well as strongly coupled optical resonances with the photonic crystal structure of diatom frustules. Second, we discovered that the evaporation-driven microscopic flow combined with the strong hydrophilic surface of diatom frustules is capable of concentrating the analyte molecules, which offers a simple yet effective mechanism to accelerate the mass transport into the SERS substrate. Using the inkjet printing technology, we are able to deliver multiple 100pico-liter (pL) volume droplets with pinpoint accuracy into a single diatom frustule with dimension around 30µm×7µm×5µm, which allows for label-free detection of explosive molecules such as trinitrotoluene (TNT) down to 10-10M in concentration and 2.7×10-15g in mass from 120nL solution. Our research illustrates a new paradigm of SERS sensing to detect trace level of chemical compounds from minimum volume of analyte using nature created photonic crystal biosilica materials.

Facile fabrication of silver nanoparticle-coated silica-C18 core–shell microspheres and their applications in SERS detection

We present a one-step method to prepare silver nanoparticle (Ag NP) shell coated functional microspheres as a surface-enhanced Raman scattering (SERS) substrate.

Recent advances in surface plasmon-driven catalytic reactions

Surface plasmons, the free electrons' collective oscillations, have been used in the signal detection and analysis of target molecules, where the local surface plasmon resonance (LSPR) can produce a huge EM field, thus enhancing the SERS signal.

A pH dependent Raman and surface enhanced Raman spectroscopic studies of citrazinic acid aided by theoretical calculations

A pH dependent normal Raman scattering (NRS) and surface enhanced Raman scattering (SERS) spectral patterns of citrazinic acid (CZA), a biologically important molecule, have been investigated. The acid, with different pKa values (~4 and ~11) for the two different functional groups (-COOH and -OH groups), shows interesting range of color changes (yellow at pH~14 and brown at pH~2) with the variation in solution pH. Thus, depending upon the pH of the medium, CZA molecule can exist in various protonated and/or deprotonated forms. Here we have prescribed the existence different possible forms of CZA at different pH (Forms "C", "H" and "Dprot" at pH~14 and Forms "A", "D", and "P" at pH~2 respectively). The NRS spectra of these solutions and their respective SERS spectra over gold nanoparticles were recorded. The spectra clearly differ in their spectral profiles. For example the SERS spectra recorded with the CZA solution at pH~2 shows blue shift for different bands compared to its NRS window e.g. 406 to 450cm(-1), 616 to 632cm(-1), 1332 to 1343cm(-1) etc. Again, the most enhanced peak at ~1548cm(-1) in NRS while in the SERS window this appears at ~1580cm(-1). Similar observation was also made for CZA at pH~14. For example, the 423cm(-1) band in the NRS profile experience a blue shift and appears at ~447cm(-1) in the SERS spectrum as well as other bands at ~850, ~1067 and ~1214cm(-1) in the SERS window are markedly enhanced. It is also worth noting that the SERS spectra at the different pH also differ from each other. These spectral differences indicate the existence of various adsorptive forms of the CZA molecule depending upon the pH of the solution. Therefore based on the experimental findings we propose different possible molecular forms of CZA at different pH (acidic and alkaline) conditions. For example forms 'A', 'D' and 'P' existing in acidic pH (pH~2) and three other deprotonated forms 'C', 'H' and 'Dprot' in alkaline pH (pH~14). The DFT calculations for these prescribed model systems were also carried out to have a plausible understanding of their equilibrium geometries and the vibrational wavenumbers. An idea about the molecular orientation of the adsorbate over nanocolloidal gold substrate is also documented.

Use of Single-Layer g-C3N4/Ag Hybrids for Surface-Enhanced Raman Scattering (SERS)

Surface-enhanced Raman scattering (SERS) substrates with high activity and stability are desirable for SERS sensing. Here, we report a new single atomic layer graphitic-C3N4 (S-g-C3N4) and Ag nanoparticles (NPs) hybrid as high-performance SERS substrates. The SERS mechanism of the highly stable S-g-C3N4/Ag substrates was systematically investigated by a combination of experiments and theoretical calculations. From the results of XPS and Raman spectroscopies, it was found that there was a strong interaction between S-g-C3N4 and Ag NPs, which facilitates the uniform distribution of Ag NPs over the edges and surfaces of S-g-C3N4 nanosheets, and induces a charge transfer from S-g-C3N4 to the oxidizing agent through the silver surface, ultimately protecting Ag NPs from oxidation. Based on the theoretical calculations, we found that the net surface charge of the Ag atoms on the S-g-C3N4/Ag substrates was positive and the Ag NPs presented high dispersibility, suggesting that the Ag atoms on the S-g-C3N4/Ag substrates were not likely to be oxidized, thereby ensuring the high stability of the S-g-C3N4/Ag substrate. An understanding of the stability mechanism in this system can be helpful for developing other effective SERS substrates with long-term stability.

Molecular imprinting: perspectives and applications

This critical review presents a survey of recent developments in technologies and strategies for the preparation of MIPs, followed by the application of MIPs in sample pretreatment, chromatographic separation and chemical sensing.

Ag Nanoparticle-Grafted PAN-Nanohump Array Films with 3D High-Density Hot Spots as Flexible and Reliable SERS Substrates

A facile fabrication approach of large-scale flexible films is reported, with one surface side consisting of Ag-nanoparticle (Ag-NP) decorated polyacrylonitrile (PAN) nanohump (denoted as Ag-NPs@PAN-nanohump) arrays. This is achieved via molding PAN films with ordered nanohump arrays on one side and then sputtering much smaller Ag-NPs onto each of the PAN-nanohumps. Surface-enhanced Raman scattering (SERS) activity of the Ag-NPs@PAN-nanohump array films can be improved by curving the flexible PAN film with ordered nanohump arrays during the Ag-sputtering process to increase the density of the Ag-NPs on the sidewalls of the PAN-nanohumps. More 3D hot spots are thus achieved on a large-scale. The Ag-NPs@PAN-nanohump array films show high SERS activity with good Raman signal reproducibility for Rhodamine 6G probe molecules. To trial their practical application, the Ag-NPs@PAN-nanohump array films are employed as SERS substrates for trace detection of trinitrotoluene and a congener of polychlorinated biphenyls. A lower detection limit of 10(-12) m and 10(-5) m can be achieved, respectively. Furthermore, the flexible Ag-NPs@PAN-nanohump array films can also be utilized as swabs to probe traces of methyl parathion on the surface of fruits such as apples. The as-fabricated SERS substrates therefore have promising potential for applications in rapid safety inspection and environmental protection.

Geometrical and morphological optimizations of plasmonic nanoarrays for high-performance SERS detection

Here we present an in-depth and comprehensive study of the effect of the geometry and morphology of nanoarray (NA) substrates on their surface-enhanced Raman scattering (SERS) performance. The high-quality SERS-active NA substrates of various unit shapes and pitches are assembled through electron beam lithography and fabricated by electron beam physical vapor deposition. Good agreement is found on comparing the Raman scattering results with the integrals of the fourth power of local electric fields from the three-dimensional numerical simulations. A novel type of hybrid NA substrate composed of disordered nanoparticles and a periodic NA is fabricated and characterized. The morphology of NAs has little influence on the SERS performance of hybrid NA substrates and they perform better than both their counterparts pure NA and disordered nanoparticle substrates.